The following disclosure is based on German Patent Application No. 100 64 143.1, filed on Dec. 15, 2000, which is incorporated into this application by reference.
1. Field of the Invention
The invention relates generally to an optical component with a low reflectance for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm at large angles of incidence, in particular between approx. 70xc2x0 and approx. 80xc2x0.
2. Description of the Related Art
In many areas the need is increasing for powerful optical components with a low reflectance and high transparency or transmission for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm. Light from this wavelength range is used for example in microlithographic exposure systems for the production of highly integrated semiconductor components with the aid of wafer steppers or wafer scanners. In the process via an illumination system a light source illuminates a mask (reticle), the image of which is reproduced with the aid of a projection system onto a photoresist coated semiconductor wafer. As it is a known fact that the miniaturization achievable with this process increases the shorter the wavelength xcex of the light used, in the most modern devices wavelengths from the deep ultraviolet range (Deep Ultraviolet, DUV) are used. Light sources for this are KrF excimer lasers with a working wavelength of xcex=248 nm and ArF excimer lasers with a working wavelength of approx. xcex=193 nm. These lasers generate linear polarized light, which, in the case of a diagonal incidence on a surface of an optical component, occurs either as s-polarized or p-polarized light, according to the individual surface orientation.
As is known the surfaces of transparent optical components are coated with so-called antireflection layers or antireflection layers (AR layers) to increase their transparency for light. Usually, in the process, multilayer systems consisting of several stacked layers of dielectric materials with various refractive indexes are used, in which layers of a high refractive material and layers of a relatively low refractive material are usually stacked alternately on top of each other.
Whereas for an effective reduction of reflection in the case of a vertical incidence of light a few layers can suffice if suitable layer materials are selected, experience shows that the number of layers required increases the bigger the angle of incidence "THgr", i.e. the angle between the direction of the incidence of light and the surface normal. This effect is shown for example in EP 0 855 604, in which the antireflection layers for UV light in the wavelength range between 150 nm and 250 nm at large angles of incidence between 70xc2x0 and 80xc2x0 are revealed. The multilayer systems proposed there are characterized in that the optical thickness of the layers of high refractive material is always the same and the optical thickness of the intermediate layers of low refractive materials is always the same, so that a periodic layer sequence results. Examples are shown for p-polarized light with a wavelength of xcex=193 nm, according to which in order to minimize the residual reflection to values of below approx. 0.5% at an angle of incidence of "THgr"=72xc2x0 seven layers are required, at an angle of incidence of "THgr"=74xc2x0 nine layers are required and at an angle of incidence of 76xc2x0 even eleven layers are required. In the case of p-polarized light with a wavelength of xcex=248 nm two additional layers each are required for the corresponding angles of incidence.
The practical use of optical components with antireflection multilayers is frequently influenced by the fact that these types of multilayer systems only show limited resistance when subjected to intensive high-energy UV radiation. As a result, the problem of the lacking laser resistance is pushed all the more to the fore the greater the energy density of the incident light. High energy densities of laser light occur for example in the field of devices for narrowing the bandwidth of excimer lasers. In U.S. Pat. No. 5,978,409 such a device is exemplarily shown, in which an arrangement of three or four prisms is provided to widen a laser beam before incidence on an echelle grate, on the hypotenuse surfaces of which the laser light always is incident with a large angle of incidence. In the case of an optimal configuration with regard to the achievable beam widening three prisms are provided, on the hypotenuse surfaces of which the UV light always impacts with angles of incidence of approx. "THgr"=74xc2x0. As for this configuration in the case of a wavelength of 193 nm, no sufficient laser resistant antireflection layer is available, uncoated prisms would have to be used, which would however lead to overall losses of more than 40% due to reflection in the case of the available substrate materials (CaF2 or synthetic quartz) and a double passage through the prisms. Therefore, as an alternative, an embodiment with four prisms is proposed, on the hypotenuse surfaces of which the laser light is incident with smaller angles of incidence between approx. 67xc2x0 and approx. 71xc2x0. To reduce the reflection a single layer of Al2O3 is always applied to the surfaces, which has sufficient laser resistance and is also intended to lead to a sufficient reduction in reflection. However, the residual reflection can not be reduced below approx. 3% via such a single layer for angles of incidence of approx. 74xc2x0.
It is an object of the invention to provide an antireflection coating for optical components, which allows an effective antireflection coating for ultraviolet light in a wavelength range between approx. 150 nm and approx. 250 nm at large angles of incidence in the range of approx. 70xc2x0 to approx. 80xc2x0 and is characterized by high laser resistance.
As a solution to this object the invention proposes an optical component having low reflectance for ultraviolet light of a wavelength in a range between approx. 150 nm and approx. 250 nm at large angles of incidence, the optical component comprising: a substrate comprising at least one surface; a multilayer system consisting of several stacked layers and applied to the at least one surface of the substrate; a layer of the multilayer system consisting of one of a high refractive dielectric material and a low refractive dielectric material; the multilayer system comprising less than five layers.
Embodiments are specified in the dependent claims. The verbatim of all claims is incorporated by reference into the subject matter of the description.
In accordance with one aspect of the invention an optical component with a low reflectance for UV light is created from the specified wavelength range at large angles of incidence by applying a multilayer system, i.e. a multilayer coating with several stacked layers, which always consists of dielectric material transparent for the UV light, to at least one surface of an optical substrate for the reduction of reflection. The layer materials are high refractive or low refractive, wherein a high refractive material has a higher refractive index in comparison with the refractive index of the other layer material and a low refractive material has a lower refractive index in comparison with the other layer material. Frequently, the refractive index of the substrate material lies between those of the layer materials. The multilayer system has less than five layers. Preferably only three or four layers are provided.
Due to the low number of layers in comparison with known multilayer systems the laser resistance of the coating can be improved only by the fact that the probability of errors leading to layer degradation in the multilayer system is usually lower, the lower the number of layers applied. The type of errors, which decrease the laser resistance, can in particular be impurities, defects or inclusions, which increase the local absorption and can thus lead to an uneven radiation load on the layer. A reduction in the number of layers leads to a process simplification, which can reduce the costs for the provision of the coated optical components according to invention.
The layer adjacent to the substrate, which is also described in the following as the first layer, preferably consists of a high refractive material so that in the case of three-layer systems with alternating high refractive and low refractive layers, the outer, third layer also consists of high refractive material, whereas in the case of four-layer systems with alternating high and low refractive material an outer layer of low refractive material is adjacent to the medium surrounding the optical component.
It is a known fact that there are only a few materials which have a sufficiently high refractive index in the considered wavelength range between approx. 150 nm and 250 nm, to allow a sufficiently large refraction coefficient ratio for an effective multilayer coating in comparison with the available low refractive layer materials. The refractive index or the refraction coefficient n of the high refractive materials for the provided wavelength is preferably at values of nxe2x89xa71.7, in particular at values of nxe2x89xa72.0. As a preference metal oxides are used as high refractive materials, which due to strong compounds have a relatively high specific laser resistance.
A particular aspect of the invention is that in the case of preferred embodiments of the antireflection coatings that serve to increase transmission, one or more layers, in particular the high refraction layers can consist of materials, which absorb the incident light to a small extent. Such materials are expediently described with a complex refractive index n=nxe2x88x92ik, wherein n is the real refractive index and k is the absorption index or the extinction coefficient at considered working wavelength. This is usually below 10xe2x88x926 in the case of the so-called non-absorbent materials. It has been shown that in order to avoid negative effects of absorption it usually suffices to select those materials in which the absorption coefficient k is greater than 10xe2x88x926 or 10xe2x88x925, but less than 0.01, in particular less than 0.005, wherein those materials are preferred, in which k is not significantly greater than 0.001.
The knowledge that slightly absorbent materials can also be used to advantage in the case of transmission increasing antireflection coatings has opened new dimensions in layer design, as the range of available materials, in particular those with a high refractive index is expanding. These advantages can also be used in multilayer systems with more than three or four layers and/or at other angles of incidence than those cited. With regard to the aspired increase in the laser destruction threshold, it should also be taken into consideration that this is indeed influenced by the absorption coefficients of the materials, but that the coating structure influencing the interference effects and the production process also influence the absorption coefficient of the coating.
In preferred embodiments of the coatings for a wavelength of 248 nm according to the invention, hafnium oxide (HfO2) is used as high refractive material. Hafnium oxide has a real refractive index n of approx. 2.1 in this wavelength range, however it also has absorption in this wavelength range and, for this reason among others, was previously not used for antireflection coatings. The absorption coefficient k is approx. 0.001. In the embodiments it is shown that when using hafnium oxide as a high refractive layer material for diagonal incident UV light with a wavelength of 248 nm a reduction of reflection of far below 0.5% residual reflection is possible, wherein the residual reflection practically disappears at an angle of incidence of "THgr"=74xc2x0. The absorption can be in the order of magnitude of approx. 0.2%, so that such coated transparent components can have a transmission coefficient greater than 99% or greater than 99.5%.
For example, zirconium oxide (ZrO2), which has an absorption coefficient in the order of magnitude of k=0.01 in the case of a real refraction coefficient n of approx. 2.2, can be used as an alternative.
In the case of multilayer systems for a wavelength of 193 nm aluminum oxide (Al2O3) is used preferably as high refractive layer material, which at this wavelength has a real refractive index in the order of magnitude of approx. n=1.7 and an absorption coefficient of approx. k=0.001. Other materials with similar optical properties can also be suitable.
The influence of absorption, in particular on the transmission and the layer heating can be kept to a minimum by keeping the overall thickness of the layers of high refractive and, if necessary, absorbent material to a minimum, for example, with overall layer thicknesses of the high refractive layers of less than 100 nm. The overall thickness can, for example, be less than 50 nm when using hafnium oxide or less than 70 nm when using aluminum oxide.
It is possible to use layer thicknesses, the optical thickness of which deviates from a quarter wavelength layer thickness (xe2x80x9cquarterwavexe2x80x9d layer thickness). If necessary, all layers of the multilayer system can have different physical thicknesses. However, multilayer systems are also possible in which the layers of the same material also have the same optical thickness.
Fluorides are used preferably as low refractive materials, in particular magnesium fluoride (MgF2). Possible alternatives such as calcium fluoride, sodium fluoride, lithium fluoride or aluminum fluoride are conceivable, insofar as the refractive index of the appropriate material is lower than that of the high refractive material and, if necessary, of the substrate material. Suitable substrate materials for transparent optical components are above all silicon oxide as glass (synthetic quartz) or single crystalline materials such as calcium fluoride or magnesium fluoride and also, if necessary, barium fluoride.
These and other features result from the description and the drawings as well as from the claims, wherein each of the individual features can always be realized individually or together in the form of sub-combinations in an embodiment of the invention and in other fields and can represent advantageous, as well as protectable embodiments.